1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting device, and, more particularly, to a high brightness nitride based semiconductor light emitting device, designed to lower operating voltage and to enhance electric current distribution effects, and a method of manufacturing the same.
2. Description of the Related Art
Generally, a nitride-based semiconductor is a material with a relatively large energy band gap (for example, in case of a GaN semiconductor, an energy band gap of about 3.4 eV), and is used for photodiodes for emitting light in a short wavelength range of blue light or green light. As for the nitride-based semiconductor, a material with the formula AlxInyGa(1-x-y)N (where 0≦x≦1, 0≦y≦1, 0≦x+y≦1) is generally used.
However, since the nitride semiconductor has a relatively large energy band gap, there is a difficulty in achieving an ohmic contact with electrodes therein. Particularly, contact resistance between a p-side electrode and a p-type nitride semiconductor is increased due to the larger energy band gap of the p-type- nitride semiconductor, thereby increasing the operating voltage of the device, which results in an increase of heat generation thereof.
Accordingly, it is necessary to provide a method for enhancing the ohmic contact when forming the p-side electrode. Additionally, since a portion formed with the p-side electrode acts as a main light-emitting surface in a typical non-flip chip structure, the above method for enhancing the ohmic contact has a technological restriction in that transmission of light must be ensured.
As to a conventional method satisfying the requirement, U.S. Pat. No. 5,563,422 entitled “Nitride gallium-based III-IV compound semiconductor device and method of manufacturing the same,” which is assigned to Nichia Chemical Industry Limited, discloses a transparent electrode layer using a bi-layer of Ni/Au. FIG. 1 shows a light emitting device according to an embodiment of the disclosure.
As shown in FIG. 1, a conventional nitride semiconductor light emitting device 10 comprises an n-type GaN nitride layer 12, a GaN/InGaN active layer 13 having a multi-well structure, and a p-type GaN nitride layer 14, which are sequentially formed on a sapphire substrate 11 in this order. In the nitride semiconductor light emitting device 10, some portion of the p-type GaN nitride layer 14 and the GaN/InGaN active layer 13 is removed to expose a portion of the n-type GaN nitride layer 13.
After forming an n-side electrode 19a on the n-type GaN nitride layer 12 and a transparent electrode 18 made of Ni/Au for forming the ohmic contact on the p-type GaN nitride layer 14, a p-side bonding electrode 19b is formed. The transparent electrode 18 is a translucent layer for enhancing contact resistance, and can be formed through a deposition process of a bi-layer of Ni/Au and a subsequent annealing treatment.
In view of formation of the ohmic contact and enhancement of electric current injection efficiency, the transparent electrode 18 is preferred over a conventional ITO electrode. However, although a NiO layer having a relatively high translucency is formed by means of the annealing treatment, the transmittance thereof is 60% at most, and thus, the transparent electrode 18 has a lower light emitting efficiency, compared with the conventional ITO electrode.
In order to ensure high transmittance, the thickness (100 μm) of the Ni/Au transparent electrode must be restricted, causing a limitation in sufficient enhancement of the electric current injection efficiency. Furthermore, in order to additionally enhance the ohmic contact, it is necessary to dope a high concentration p-type impurity, such as Zn, Be, Mg and Cd, on the top surface of the p-type GaN nitride layer. However, an excessive amount of the p-type impurity causes reduction of crystallinity.
Accordingly, it is necessary to provide a new light emitting device, which can sufficiently enhance the ohmic contact and electric current distribution efficiency even with the conventional ITO electrode, and a method of manufacturing the same.